The invention relates to a method for the production of an electrically insulating base material for the manufacture of perforated plated-through printed circuit panels with a metal core.
The use of panels of insulating material with a metal core as base material for printed circuit panels is known. The metal panel serving as the core may consist, for example, of iron or (preferred on account of its low weight and its good thermal conductivity) of aluminum. If the panel is to be provided with holes for making plated-through holes, the latter are in general produced, for example, drilled, even before the insulating material is applied. Uniform and pore-free insulation, especially of the hole walls, is an essential requirement for the usability of such products. Because of the required heat removal, the insulating layers should be as thin as possible, it being essential, however, that the freedom from pores is maintained. Many methods are known which are concerned with this problem.
According to U.S. Pat. No. 3,296,099 a metal panel provided with prepunched holes is coated with an epoxy powder by the fluidized bed sintered method. In this process the entire surface including the hole walls are coated with epoxy resin. A disadvantage is the hourglass-like or egg-timer-like shape of the insulating layer inside the holes; towards the center of the panel the coating is always thicker than at the ends of the holes. In order to insulate the hole edges adequately it is necessary to accept very thick insulating layers in the region of the hole center. It is therefore necessary to start from relatively large hole diameters in the metal core. As a result of this only a relatively low surface utilization is in turn possible in the construction of printed circuit panels. The trend of modern printed circuit panel techniques is, however, towards printed circuit panels with everincreasing conductor track density.
In German Offenlegungsschrift No. 2,453,788 a method is described in which holes in aluminum panels are filled with loose thermoplastic and thermosetting plastic material and the material is then compacted into solid plugs which completely fill the holes. Narrow holes are then drilled in these plugs so that a hole wall coated with insulating material is left. This method certainly yields cylindrical holes, but is complicated and expensive.
In German Pat. No. 1,954,973 a method is described in which the predrilled aluminum panels are first provided with a first coat of insulating material and coated by spraying with the actual thermosetting material in 3 to 10 layers lying on top of each other and then the coatings are thermally cured. In this method, just as in the fluidized bed sintering method, the insulating material first softens on heating, it flows and becomes thinned at the hole edges (hourglass effect). This can only partially be compensated for by the multiple coatings, and the coating process becomes very expensive as a result of this.
A further known coating method is electrostatic spraying. In this case there are produced at the hole walls layers whose thickness decreases with the distance from the hole edge since the electric field is screened by the metal of the panel. The effect becomes all the more pronounced the thicker the metal panel and the more restricted the hole diameter are. The insulating material coatings must, however, be pore-free. This requirement can be better fulfilled with increasing layer thickness. However, it is in the nature of the electrostatic spray coating method that uniform and, in particular, pore-free coatings are not obtained in the holes.
The electrophoretic coating method, which is also known, results in relatively pore-free and uniform coatings. However, to make possible the migration of the particles to the electrode, it is necessary for them to be charged with ions. As a result of this, however, the insulating properties of the coatings, in particular the bulk and surface resistance, are reduced.
Electrophoretic coating of anodized aluminum with organometallic thermosetting compounds is described in European Patent Application No. 58,023. Since the compounds used in this case contain no ionic groups which produce conductivity, it is necessary to use high voltages and long coating times. In this case, for example, 500 to 10,000 volts and 30 to 120 minutes are specified. This is very expensive and hazardous.
Incidentally, it is very difficult to bond the deposited conductor track metal firmly to the said insulating materials. Etching with chromic acid as is otherwise usual is of litle effect for the substances used here.
According to German Offenlegungsschrift No. 3,047,287, to improve the anchoring of the conducting metal to the insulating material surface, an anchoring or adhesive material layer is applied. This layer is applied only to the flat principal faces of the base material, not to the surfaces of the hole walls. The metal layer therefore exhibits low adhesion at the walls, which may be disadvantageous particularly in the soldering process. The reason for this procedure lies in the fact that at the stage of manufacturing the base material the position of the holes for the circuit is not known and the subsequent coating of the hole walls with adhesive is difficult and expensive.
For-the above-stated reasons, perforated printed circuit boards with a metal core have not yet become established in the industry although a technical requirement exists for them. In particular it would be desirable to exploit the high thermal and mechanical strength achievable with such boards and also the good thermal conductivity, which permits a denser arrangement of tracks and components.